Photographic emulsions, elements, and processes



Unite States This invention relates to photographic gelatinosilverhalide emulsions and emulsion layers and to photographic elementsembodying the same. More particularly, it rehates to such emulsionlayers and elements having improved photographic characteristics. Stillmore particularly, the invention relates to a process for increasing.

the covering power of developed silver and improving the maximumdensity, contrast-and effective speed of gelatino-silver halideemulsions.

It:i-s known to add certain chemical compounds, .e.g., heavy metal andnoble" metal salts to improve the sensitomeric characteristics of Iphotographic gelatino-silver halidecmulsions. a practical. limit to-the.advantages which can be obtained bythe addition of suchcompounds withoutaccompanyingg deleterious eiiects such as uncontrollable fog andinstability on tropical aging.

It-has been proposed to add to, or replace all or part of-theigelatin inthe gelatino-silver halide system with various polymeric colloidmaterials for various reasons including attempts to overcome thewell-known disadvantages of gelatin. However, a few of these proposalshave overcome these disadvantages in a satisfactory manner, and gelatinis still-used in most commercial photographic-films. In general, whenall ofthe gelatin is-replaced by the synthetic polymers suggested by theprior art, many of the-outstanding photographic and colloidchemicalproperties of gelatin are lost.

An object of this invention is to provide improved r Still'furtherobjects will be apparent from the following description of theinvention.-

It has now beenfounthin' accordance with this invention,.thatg'elatino-silver halide emulsions of increased covering power andenhanced properties can be made by adding to the emulsions an amount ofat least one water-soluble pectic polysaccharide taken from the groupconsistingrof araba'n, arabogalactan and galactan and havingarr-"average molecular weight of about 6,000 to 100,000 so that thepectic polysaccharide constitutes 20 to 80 parts and preferably 30 to 70parts per 100 parts of gelatin in. a gelatino-s'ilver halide emulsion.The amount of pectic polysaccharide present is based on finishedemulsion ready for coating onto a film, paper, metal foil, glass plateor other support.

The pectic polysaccharides, araban, galactan and aminogalactan, arepolymers of L-arabinose and D-galactose and are present in the tissuesand seeds of the higher orderof plants from which they may be extracted.

In general, the finished emulsion will contain 20 to 80 parts of one ormore of the pectic polysaccharides per 100 parts of gelatin and thegelatin may be in a ratio to silver halide of from 1:06 to -1':1.5 (60to 150 parts by weight per 100 parts by weight of gelatin). Theemulsions may, of course, contain small amounts of conventionaladjuvants. The final emulsions show markedly increased silver coveringpower (e.g., to 40% greater) There is, however, at the present time,

atent ree over all gelatin emulsions. To' be more specific, with respect to coveringpower for the same quantity of silver halide, largeincreases in maximum density and contrast can be obtained in thedeveloped image. Covering power of developed silver can be-expressed asthe numerical result of dividing optical density by the grams of silverper square decimeter in the developed image layer. The

covering power will, of course, vary with the amount of" utilized in anygelatinosilver halide system to improve its efliciency. The inventionisalso very useful in the socalled graphic arts films, i.e.,lithographic films. In all cases when the pectic polysaccharide ispresent in the amounts set forth above, a higher density is obtainedfrom a given amount of metallic silver in the developed photographiclayer, although the effect is more noticeable in large-grain silverhalide emulsions than in smallgrain emulsions.

The processes of the invention'are quitesimple and consist ofrr'ierely'admixi'rig with'the" molten or liquefied emulsion, after thedigestion step, an aqueous solution of one or more of the water-solublepectic polysaccharides. dobromide emulsion useful for X-ray films isprepared by conventional methods. The silver halides are precipitated ingelatin and the resulting dispersion or'emulsion ripened. The emulsionis then washed, either after chilling and noodling, or aftercoagulation, by decanting the supernatant liquid. The emulsion isredispersed and then it is digested to bring it to maximum speed.Following digestion, a water-soluble pectic polysaccharide having anaverage molecular weight of 6,000 to 100,000 (usually in aqueoussolution) is added to the molten emulsion in an amount sufficient toprovide the desired ratio of pectic polysaccharide to gelatin. 7 Theusual final additions, e.g., of hardener, anti-fogging agents,sensitizing agent and spread-ing agent, are made and the emulsion iscoated on a suitable support and dried inthe usual manner. To test theeffectiveness of the-pectiopolysaccharide the element is exposed in asensitometer accord ing to a procedure based on the American StandardMethod for the Sensitometry of Medical X-ray Films PH 2.9-1956."

The sensitometer used in' the following examples was equipped with aneutral density /2 step wedge. The density of. a selected step, as setforth in the tables, was measured and the quantity of metallic silverper square decimet'er was determined by analysis. The dens= ity dividedby the quantity of silver in grams per square decirneter was taken as anexpression of covering power of the developed silver.

The invention will now be illustrated further by, but is not intended tobe limited to, the following examples. The quantities of the pecticpolysaccharides are given as parts by weight per parts" by weight ofgelatin in the emulsion.

Example I A high speed gelatino-silver iodobromide emulsion was made inthe usual manner and digested with an organic sulfur compound and goldchloride. It contained approximately 1.6 mol percent of silver iodideand 98.4 mol percent of silver bromide and the ratio 015 gelatin to Asan exemplary procedure, a gelatino-silver io- Gramsp-N-methylaminophenol hydrosulfate 3.0 Anhydrous sodium sulfite 50.0Hydroquinone 9.0 Anhydrous sodium carbonate 50.0 Potassium bromide 4.5

Water to make 1,000 ml.

Following development, the films were fixed, washed and dried in theconventional manner. The results of the sensitometric tests andmeasurements of covering power are shown in the following table:

Parts of Covering Arabo- Power of galactan Maximum Developed per 100Density Silver at parts of Densities Gelatin o1 1.33:.1

1 (Control) 1.23 29 2 17 1.76 37 3 35. 5 1. 78 36 Example 11 An emulsionwas made and treated in the same manner as in Example I except that a 5%aqueous solution of the pectic polysaccharide, araban, an extract ofapple pectin having an average molecular weight 6,500, was added inplace of the arabogalactan. The results are shown in the followingtable. The relative coating weights of silver halide are given to showthat the last emulsion containing araban was coated at 74% of thecoating weight of the control.

Covering Parts of Power of Arabian Relative Maximum Developed Per 100Coating Density Silver at parts of Weight Densities Gelatin of 1.3 :l:.1

1 (Control) 0 100 1. 23 29 17 103 1. 37 31 35. 5 106 1. 49 33 68 74 1.21 35 It will be seen that, although substantially less sliver halidewas used in the last coating containing araban, as indicated by therelative coating weight, the maximum density obtained was close to thatof the control by virtue of the higher covering power of the developedsilver. Substantial savings in silver cost can be realized by this moreefficient use of developed silver.

It has been found that the increased covering power of the silver indeveloped photographic emulsion layers is not limited to adding thewater-soluble pectic polysaccharides to the gelatino-silver halideemulsion layer. The beneficial results also can be attained byincorporating the pectic polysaccharides in a gelatin composition to becoated next to a gelatino-silver halide emulsion layer, e.g., in agelatin sublayer, a separator or lightfiltering layer or in anantiabrasion layer. The amount of araban, galactan, arabogalactan ormixtures thereof so used can be based on the total gelatin in thecontiguous layers. A layer essentially composed of gelatin and one ormore of the pectic polysaccharides in an amount of 20 to 80 parts per100 parts by weight of the total gelatin in the contiguous layers can beused.

As will be apparent from the above description, the invention is notlimited to the specific quantities or types of water-soluble pecticpolysaccharides as shown in the examples. According to the technicalreference Polysaccharide Chemistry by Whistler and Smart, AcademicPress, Inc., New York 1953, pages 198-207, the pectic polysaccharides ofthis invention are composed of 01,4- D-galactopyranose units, i.e.,galactan; approximately l--% 5-l,6'-D-galactopyranose and[i-1,3'-D-galactopyranose units, and approximately 12% terminal L-arabofuranose units, i.e., arabogalactan; and a-1,5-L- arabofuranoseunits and a-1,3-L-arabofuranose units, i.e., araban. The lattermaterial, araban, is a polymer of L-arabinose which can be extractedfrom apple pectin and peanuts. Generally, araban contains small amountsof pectic acid and galactan. Galactan is present in numerous planttissues and can be obtained in reasonable purity from the seeds of theshrubby plant, white lupine. The arabogalactan can be extracted fromlarch wood.

The invention is not limited to photographic gelatinosilver halideemulsions of the silver iodobromide type. The invention may be appliedto other gelatino-silver halide emulsions e.g., gelatino-silverbromochloride emulsions of the lithographic type as well as silverbromide emulsions. This invention is particularly efficacious inphotographic emulsions where the average silver halide grain size isrelatively large.

The pectic polysaccharides are useful not only with black and whitephotographic and X-ray emulsions, but with photographic emulsions usedin color photography. Suitable emulsions can contain color-formers inaddition to gelatin and at least one of the water-soluble pecticpolysaccharides. Examples of useful gelatino-silver halide emulsions andcolor-formers useful therein are disclosed in Middleton and Jennings US.2,319,426; Dorough, U.S. 2,380,032, Dorough, US. 2,380,033 and Woodwardand C'hu, US. 2,927,024. In the case of emulsions containing pecticpolysaccharides which are to be used for color photography, the emulsionlayers may be developed in color developers containing thecolor-formers. The invention may also be applied to photographicelements useful in dye imbibition and wash-01f reliefi processes forcolour photography.

The emulsions may contain any of the well-known optical sensitizing dyesas well as non-optical sensitizers such as sulfur sensitizers containinglabile sulfur, e.g., allyl isothiocyanate, allyl diethyl thiourea,phenyl isothiocyamate and sodium thiosulfate, the polyoxyalkylene othersin Blake et al. US. Patent 2,400,532 and the polyglycols disclosed inBlake et al. US. Patent 2,432,549. Other optical sensitizers such asamines as taught by Staud et al. U. S. Patent 1,925,508 and metal saltsas taught by Baldsiefen US. Patent 2,540,085 and Baldsicfen et al. US.

Patent 2,540,086 may also be used. Antifoggants, e.g., benzotriazole andtriazaindenes, can be used as well as the usual hardeners, i.e., chromealum, formaldehyde, etc. Pigments, matting agents and other well-knownphotographic emulsion adjuvants also may be used in conjunction with thepectic polysaccharides.

The emulsion may be coated on any suitable support such as paper orfilms composed of cellulose esters, e.g., cellulose triacetate,cellulose acetate/butyrate; superpolymers, e.g., polyvinyl chloride (co)vinyl acetate; polyvinyl acetals, e.g., formals, acetals; polystyrene;polyamides, e.g., polyhexamethylene adipamide, and polyesters, e.g.,polyethylene terephthalate, polyethylene terephthalate/isophthalate,esters formed by condensing terephthalic acid and dimethyl terephthalatewith propylene glycol, diethylene glycol, tetramethylene glycol orcyclohexane-l,4-dimethanol (hexahydro-p-Xylene alcohol). The vinylidenechloride copolymer-coated oriented polyester films of Alles et al. US.Patent 2,627,088 and Alles US. Patent 2,779,684 are especially suitable.

An advantage of the invention is that it provides a simple, dependableand effective means for providing gelatino-silver halide photographicemulsions of enhanced covering power 015 developed silver. Theefticiency of the resulting developed silver is increased, that is, itcan provide greater density per quantity of metallic silver whichresults from development. I

Another advantage of the invention is that the addition of pecticpolysaccharides to photographic emulsions requires no special techniqueand can he carried out by the ordinary technician with conventionalappacatus. Since :the pectic polysaccharides can he added from aqueoussolutions, it does not provide the problems of solvent recovery and theresulting modified gelatino-silver halide emulsions can be coated anddried in the conventional coating and drying apparatus which ofierscommercial advantages. Still \iurther advantages will be apparent fromthe foregoing description of the invention.

What is claimed is:

1. A photographic silver halide emulsion having increased covering powerof developed silver comprising, on a dry basis per 100 parts by weightof gelatin, 20 to 80 parts by weight of at least one water-solublepectic polysaccharidetaken from the group consisting of araban,arabogalactan and galact-an having a molecular weight of 6,000 to100,000, and 60' to 150 parts by weight of a light-sensitive silverhalide.

2. An emulsion as defined in claim 1 wherein said pectic polysaccharideis ama bogalactan.

3. An emulsion as defined in claim 1 wherein said silver halide issilver iodobromide.

4. A photographic element having increased covering power ofi developedsilver comprising a support and a gelatino-lsilver halide emulsion layercomprising, on a dry basis per 100 parts by weight of gelatin, 20 to 80parts by weight of at least one water-soluble pectic polysaccharidetakenfrom the group consisting of araban, arabogalaotan and galactanhaving an average molecular weight of 6,000 to 100,000, and 60 to 150pants by weight of a light-sensitive silver halide. t

5. An element as defined in claim 4 wherein said pectic polysaccharideis arabogalactan.

6. An element as defined in claim 4 wherein said silver halide is silveriodobromide. t

7. A photographic element having increased covering power of developedsilver comprising a support, a gelatino-silver halide emulsion layer anda layer contiguous the silver halide emulsion layer, the said contiguouslayer containing, on a dry basis per 100 pants by weight of totalgelatin in said layers, 20 to 80 parts by weight of at least onewatersoluble pectic polysaccharide taken irom the group consisting ofaraban, arahogalactan and galactan having a molecular weight of 6,000 to100,000, any remaining constituent of the contiguous layer beinggelatin.

8. A process for increasing the covering power of developed sil-verwhich comprises admixing with a gelatinosilver halide emulsioncontaining, on a dry weight basis per 100 parts by weight of gelatin, to150 parts by weight ofi light-sensitive silver halide, from 20m parts byweight of at least one water-soluble pectic polysaccharide taken fromthe group consisting of anaban, a-rabogalactan and galactan per partslay weight of gelatin, said pectic polysacoharide having an averagemolecular Weight of 6,000 to 100,000.

9. A process as defined in claim 8 wherein said pectic polysaccharide isarabogal-actan.

10. A process as defined in claim 8 wherein said silver halide is silveriodobromide.

References Cited in the file of this patent UNITED STATES PATENTSNicholes et al May 11, 1948 lllingsworth et a1 Oct. 10, 1961 Whistler etal: Polys-accharide Chemistry, Academic Press, Inc, New York, 1953,pages 198-207.

1. A PHOTOGRAPHIC SILVER HALIDE EMULSION HAVING INCREASED COVERING POWEROF DEVELOPED SILVER COMPRISING ON A DRY BASIS PER 100 PARTS BY WEIGHT OFGELATIN, 20 TO 80 PARTS BY WEIGHT OF AT LEAST ONE WATER-SOLUBLE PECTICPOLYSACCHARIDE TAKEN FROM THE GROUP CONSISTING OF ARABAN, ARABOGALACTANAND GALACTAN HAVING A MOLECULAR WEIGHT OF 6,000 TO 100,000, AND 60 TO150 PARTS BY WEIGHT OF A LIGHT-SENSITIVE SILVER HALIDE.